This invention relates to the design of optical coatings for reducing the amount of light which is reflected from an optical surface.
Modification in the intensity of light which may occur when two or more beams of light are superposed is known as interference. The principle of superposition states that the resultant amplitude is the sum of the amplitudes of the individual beams. The brilliant colors, for example, which may be seen when light is reflected from a soap bubble or from a thin layer of oil floating on water are produced by interference effects between two trains of light waves. The light waves are reflected at opposite surfaces of the thin film of soap solution or of oil.
One important practical application for the principles of interference in thin films involves the production of coated optical surfaces. If a film of a transparent substance is deposited on glass, for example, with a thickness which is one quarter of a particular wavelength of light in the film, the reflection of that light from the glass surface can be almost completely suppressed. The light which would otherwise be reflected is not absorbed by a nonreflecting film; rather, the energy in the incident light is redistributed so that a decrease in reflection is accompanied by a concomitant increase in the intensity of the light which is transmitted.
Nonreflecting films are of practical importance because they can be used to greatly reduce the loss of light by reflection at the various surfaces of an optical system. Stray light, which could otherwise reach the image because of these reflections, can also be substantially eliminated, with a resulting increase in contrast. Such improvements are particularly useful where an image is formed by a highly corrected lens system which employs a large number of air-glass surfaces. Consequently, almost all optical components of high quality are coated to reduce reflection. These coatings were first made by depositing several monomolecular layers of an organic substance on glass plates. More durable coatings may be fabricated by evaporating calcium or magnesium fluoride on the surface in a vacuum, or by chemical treatment with acids which leave a thin layer of silica on the surface of the glass.
Considerable improvements have been achieved in the antireflective performance of such films by using a composite film having two or more superimposed layers. The use of gradient index layers, in which the index of refraction within the layer is made to vary continuously as a function of depth in the layer, further increases the degrees of freedom available in the design of such films. Modern applications of optical technology, however, require antireflective films with even lower levels of reflection than have previously been attainable in the art. One of the ways in which higher performance antireflective layers have been obtained is through utilizing digital equivalents of continuous gradient index layers. The spectral performance of an arbitrary generalized gradient index interference coating may be closely approximated by some digital configuration which is a sequence of thin layers each having a high or a low refractive index. It is the goal of this invention to further advance the performance of antireflective coatings by providing a new technique for defining an optimized digital index profile for a film with a given thickness and spectral performance.